1. Field of the Invention
The present invention is directed, in general, to printing heads for nonimpact printers and, more particularly, to a method of electrically and mechanically joining separate piezoelectric members into an ink jet printhead having an array of ink-jecting channels using electromagnetic induction heating.
2. Description of Related Art
Printers provide a means of producing a permanent record in human-readable form. Typically, a printing technique may be categorized as either impact printing or non-impact printing. Impact printing is typically effected by striking a ribbon placed near the surface of the paper to receive the print. Impact printing techniques may be further characterized as either formed-character printing or matrix printing. In formed-character printing, the element that strikes the ribbon to produce the image consists of a raised mirror image of the desired character. In matrix printing, the character is formed as a series of closely-spaced dots that are produced by striking a provided wire or wires against the ribbon. By selectively striking the provided wires, any character representable by matrix of dots can be produced.
Non-impact printing is often preferred over impact printing in view of its tendency to provide higher printing speeds as well as its better suitability for printing graphics and half-tone images. Non-impact printing techniques include matrix, electrostatic and electrophotographic printing techniques. In matrix printing, wires are selectively heated by electrical pulses and the heat thereby generated causes a mark to appear on a sheet of paper, usually a specially treated paper. In electrostatic printing, an electric arc between the printing element and the conductive paper removes an opaque coating on the paper to expose a sub-layer of a contrasting color. Finally, in electrophotographic printing, a photoconductive material is selectively charged using a light source such as a laser. A powder toner is attracted to the charged regions and, when placed in contact with the sheet of paper, transfers to the paper's surface the powder toner. The toner is then subjected to heat that fuses it to the paper in the desired image.
Another form of non-impact printing is generally classified as ink jet printing. Ink jet printing systems use the ejection of tiny droplets of ink to produce an image. The devices produce highly reproducible and controllable droplets that are ejected precisely at a right time and velocity to produce a desired image on the paper. Most ink jet printing systems commercially available may be generally classified as either a "continuous jet" type ink jet printing system where droplets are continuously ejected from the printhead and either directed to or away from the paper depending on the desired image to be produced or as a "drop on demand" type ink jet printing system where droplets are ejected from the printhead in response to a specific command related to the image to be produced.
Continuous jet type ink jet printing systems are based upon the phenomenon of uniform droplet formation from a stream of liquid issuing from an orifice. It has been previously observed that fluid ejected under pressure from an orifice about 50 to 80 microns in diameter tends to break up into uniform droplets upon the amplification of capillary waves induced onto the jet, for example, by an electromechanical device that causes pressure oscillations to propagate through the fluid. Due to the small size of the droplets and the precise trajectory control, the quality of continuous jet type ink jet printing systems can approach that of formed-character impact printing systems. However, one drawback to continuous jet ink jet printing systems is that fluid must be jetting even when little or no printing is required. This requirement degrades the ink and decreases reliability of the printing system.
Due to this drawback, there has been increased interest in the production of droplets by electromechanically induced pressure waves. In this type of system, a volumetric change in the fluid is induced by the application of a voltage pulse to a piezoelectric material that is directly or indirectly coupled to the fluid. This volumetric change causes pressure/velocity transients to occur in the fluid and these are directed so as to produce a droplet that issues from an orifice. Since the voltage is applied only when a droplet is desired, these type of ink jet printing systems are referred to as drop-on-demand. For example, in FIG. 1, a drop-on-demand type ink jet printer is schematically illustrated. A nozzle assembly 306 draws ink from a reservoir (not shown). A driver 310 receives character data and actuates the piezoelectric material 308 in response thereto. For example, if the received character data requires that a droplet of ink is to be ejected from the nozzle assembly 306, the driver 310 applies a voltage to the piezoelectric material 308. The piezoelectric material 308 then deforms in a manner that forces the nozzle assembly 306 to eject a droplet of ink from the orifice 312. The ejected droplet then strikes a sheet of paper 318.
The use of piezoelectric materials in ink jet printers is well-known. Most commonly, piezoelectric material is used in the piezoelectric transducer by which electric energy is converted into mechanical energy by applying an electric field across the material, thereby causing the piezoelectric material to deform. This ability to distort piezoelectric material has often been used to force the ejection of ink from ink-carrying channels of ink jet printers. One such ink jet printer configuration that uses the distortion of a piezoelectric material to eject ink includes a tubular piezoelectric transducer that surrounds an ink-carrying channel. When the transducer is excited by the application of an electrical voltage pulse, the ink-carrying channel is compressed and a drop of ink is ejected from the channel. For example, an ink jet printer that uses circular transducers may be seen in U.S. Pat. No. 3,857,049 to Zoltan. However, the relatively complicated arrangement of the piezoelectric transducer and the associated ink-carrying channel causes such devices to be relatively time consuming and expensive to manufacture.
To reduce the per ink-carrying channel (or "jet") manufacturing costs of an ink jet printhead, in particular, those ink jet printheads having a piezoelectric actuator, it has long been desired to produce an ink jet printhead having a channel array in which the individual channels that comprise the array are arranged such that the spacing between adjacent channels is relatively small. For example, it would be very desirable to construct an ink jet printhead having a channel array where adjacent channels are spaced between approximately 4 and 8 mils apart. Such an ink jet printhead is hereby defined as a "high density" ink jet printhead. In addition to a reduction in the per ink-carrying channel manufacturing costs, another advantage that would result from the manufacture of an ink jet printhead with a high channel density would be an increase in printer speed. However, the very close spacing between channels in this high density ink jet printhead has long been a major problem in the manufacture of such printheads.
Recently, the use of shear mode piezoelectric transducers for ink jet printhead devices has become more common. For example, U.S. Pat. Nos. 4,584,590 and 4,825,227, both to Fischbeck, et al., disclose shear mode piezoelectric transducers for a parallel channel array ink jet printhead. In both of the Fischbeck, et al. patents, a series of open ended parallel ink pressure chambers are covered with a sheet of piezoelectric material along their roofs. Electrodes are provided on opposite sides of the sheet of piezoelectric material such that positive electrodes are positioned above the vertical wall separating pressure chambers and negative electrodes are positioned over the chamber itself. When an electric field is provided across the electrodes, the piezoelectric material, poled in a direction normal to the electric field direction, distorts in a shear mode configuration to compress the ink pressure chamber. In these configurations, however, much of the piezoelectric material is inactive. Furthermore, the extent of deformation of the piezoelectric material is small.
An ink jet printhead having a parallel channel array and that uses piezoelectric materials to construct the sidewalls of the ink-carrying channels may be seen in U.S. Pat. No. 4,536,097 to Nilsson. In Nilsson, an ink jet channel matrix is formed by a series of strips of piezoelectric material disposed in spaced-apart parallel relationship and covered on opposite sides by first and second plates. One plate is constructed of a conductive material and forms a shared electrode for all of the strips of piezoelectric material. On the other side of the strips, electrical contacts are used to electrically connect channel-defining pairs of the strips of piezoelectric material. When a voltage is applied to the two strips of piezoelectric material that define a channel, the strips become narrower and higher such that the enclosed cross-sectional area of the channel is enlarged and ink is drawn into the channel. When the voltage is removed, the strips return to their original shape, thereby reducing channel volume and ejecting ink therefrom.
An ink jet printhead having a parallel ink carrying channel array and that uses piezoelectric material to form a shear mode actuator for the vertical walls of the channel has also been disclosed. For example, U.S. Pat. Nos. 4,879,568 to Bartky, et al. and 4,887,100 to Michaelis, et al. each disclose an ink jet printhead array in which a piezoelectric material is used as the vertical wall along the entire length of each channel in forming the array. In these configurations, the vertical channel walls are constructed of two oppositely poled pieces of piezoelectric material mounted next to each other and sandwiched between top and bottom walls to form the ink channels. Once the ink channels are formed, electrodes are deposited along the entire height of the vertical channel wall. When an electric field normal to the poling direction of the pieces of piezoelectric material is generated between the electrodes, the vertical channel wall distorts to compress the ink jet channel in a shear mode fashion.
Finally, U.S. Pat. No. 5,227,813 to Pies, et al. is directed to a sidewall actuated channel array for a high density ink jet printhead. The sidewall actuator includes a top wall, a bottom wall and at least one elongated liquid confining channel defined by the top wall, the bottom wall and sidewalls. The actuator sidewall is comprised of a first actuator sidewall section formed of a piezoelectric material poled in a first direction perpendicular to a first channel and attached to the top wall, a second actuator sidewall section attached to the first sidewall section and the bottom wall and means for applying an electric field across the first actuator sidewall section and perpendicular to the direction of polarization. When the electric field is applied across the first sidewall section, the actuator sidewall engages in a motion that produces an ink-ejecting pressure pulse in the channel. The printhead configuration shown in Pies, et al. is referred to as a "double-u" configuration wherein each channel has a joint between piezoelectric pieces at a point in the sidewalls. Pies et al. discloses use of a conductive epoxy to join the various pieces of piezoelectric material together. The other ink jet printheads described above similarly use metal-bearing epoxy, epoxy with conductively-coated microbeads or epoxy preforms to join materials together.
A metal-bearing or microbead epoxies rely on making electrical contact between a plurality of metal pieces entrained within the substantially non-conducting epoxy material. While the epoxy material is used to make mechanical contact between the piezoelectric members, the metal pieces within the epoxy is used to make electrical contact. It is important in ink jet printheads to have a uniform electrical and mechanical bond between the various pieces making up the ink jet printhead. In many cases, and particularly for printheads, it is necessary to produce a minimal thickness bond of the order of several micrometers possessing reliably low electrical resistance normal to, and in the plane of, the bond. Thinness and a desirably high shear modulus of the material constituting the bond are difficult to obtain by traditional methods. The modulus of epoxy is considerably less than that of solders, but the conventional method of applying solder paste by stencil or screen methods have inherent practical limits on the thinness of the bond that can be produced.
What is needed in the art is a method of manufacturing an ink jet printhead that does not require use of a conductive epoxy, with all of its attendant manufacturing and operational disadvantages. The art also lacks a method of bonding piezoelectric materials together that is fast and does not harm the physical qualities of the piezoelectric material.